ok, the truck is a 90 model chevy with a 92 cummins swapped in. the problem it started doing is the engine started to run a little rough. it kinda comes and goes. sometimes it will rev smooth, it will take off and has plenty of power. here lately (which has been more frequent ) the engine will idle rough and real low. when u rev it it will be rough down low and seems to smooth up in the higher rpms. now sometimes when just sitting there idle or after lifting off the peddle it will just die. the lift pump looks new, and i just replaced the fuel filter. it has had a gov spring and fuel pin put in. seems to have plenty of fuel, it will smoke black when u step on it and im getting about 30 psi on boost. im not sure where to look on the problem and i dont want to start parts changing unless i know for sure. im wonderimg if its in the injection pump or injectors? any help would be appreciated, thanks

 

I would do the overhead on it (slash the valves). Also you can pull your injectors and have them flow matched and pop tested, just to make sure they are popping at the correct pressure and that one or more is not injecting more or less then the others.

 

Air leak in the fuel line somewhere.